What Really Saves Gas
Gas saving tips tested -- with amazing results.
*** By Edmunds.com Editors ***
Gas-saving tips have an urban legend appeal.
Someone at a party might say, "Hey, did you know that if you drive with your windows up and the air conditioning on, it actually saves gas? It's more aerodynamic."
"Huh," you say, "I didn't know that. That's interesting."
Interesting, yes. True? Well, maybe not.
We took the top four fuel economy tips and put them to a real-world test. Our goal was simple: to see what tips produced a measurable difference in fuel economy. We say "measurable" (meaning detectable by an ordinary driver, not a lab technician) because most people want to see an improvement in their fuel economy that saves dollars, not just pennies.
Fuel economy tips seem to get passed on from person to person until they lose their source and their validity. This was our chance to reverse the trend.
We chose four fuel economy tips and took two cars from the Edmunds.com long-term fleet, and drove eight 56-mile loops. Our route circled Owens Lake near Lone Pine, California, at the foot of Mount Whitney, the tallest mountain in the lower 48 states. We chose the route because it was so deserted we could vary our speed and driving style without interfering with the flow of traffic. The only other cars we saw on the route were a group of testers from Mercedes-Benz, trying out a heavily cladded prototype.
We drove the Edmunds.com long-term 2005 Ford Mustang GT and the 2005 Land Rover LR3 SE. The Mustang, with a manual transmission, is a sleek coupe; the Land Rover, with an automatic, is a boxy SUV. Both vehicles have V8 engines. The Land Rover is rated at 14 mpg city and 18 mpg on the highway according to the EPA estimates posted on Edmunds.com. The Mustang's EPA estimated mileage is 15 mpg city and 25 on the highway.
In earlier studies, we have found that the EPA estimates do not reflect the mileage that most drivers will actually record. In "Real World vs. EPA Estimates", we found that most drivers will actually get less than the city mileage.
The Land Rover has an onboard computer to calculate several driving factors including gas mileage, distance traveled and average speed. We felt the results in the Land Rover were extremely accurate. After each loop we refueled the Mustang at the same filling station and at the fuel pump to see how gas mileage varied.
Since the tests were to compare the difference between variables, we knew that we had to standardize them as much as possible. Consequently, we always drove the loops back-to-back to minimize differences in the temperature and wind direction. The two cars were driven in tandem separated by about a tenth of a mile. Results were measured and computed immediately after the loops were driven.
Test #1: Effect of Using Cruise Control
Result: Big-time savings!
Here's a bit of advice that surfaces frequently on tips lists. Usually, the recommendation is to use cruise control "selectively." This means not to use it in the mountains since it will try to keep the car up to speed no matter what grade you are climbing. We have always agreed with this tip in theory but we hadn't expected such significant results. As soon as we began driving the loop we realized it would be important. First, it smoothes out the driver's accelerator input by keeping nervous drivers from "surging." Second, it forces the driver to take the long view of the road instead of reacting to every change in the traffic.
Method: We drove the two test cars 56.8 miles once with the cruise control on and once by controlling the car manually. With the cruise control on we set it at 70 mph. With the cruise control off we varied our speed between 65 mph and 75 mph. We tried to mimic the driving style of a person who is in moderate freeway traffic.
With cruise control 19.6 mpg
Without cruise control 17.2 mpg
Percent change 13.9 percent improvement
With cruise control 23.3 mpg
Without cruise control 22.3 mpg
Percent change 4.5 percent improvement
Test #2: Effect of A/C On, Windows Up vs. A/C Off, Windows Down
This has got to take you back to the days with the family on vacation. Dad says, "Turn the A/C off! It wastes gas!" And Mom says, "We can't roll the windows down or everyone on the highway will think we can't afford A/C." And you're in the back roasting, hoping someone will win the argument so you can cool off.
Well, family psychology aside, Dad wasn't necessarily right. While the A/C compressor does pull power from the engine wasting some gas, the effect appears to be fairly minimal in modern cars. And putting the windows down tends to increase drag on most cars, canceling out any measurable gain from turning the A/C off. But this one depends on the model you're driving. Still, in our experience, it's not worth the argument because you won't save a lot of gas either way. So just do what's comfortable.
Method: We drove the same loops at equal speeds both times, 65 mph. The first loop we drove with the A/C on and the windows up. The second loop we drove with the A/C off and windows down.
With A/C on, windows up 19.3 mpg
With A/C off, windows down 19.6 mpg
Percent change 1.6 percent improvement
With A/C on, windows up 29.5 mpg
With A/C off, windows down 30.7 mpg
Percent change 4.1 percent improvement
Test #3: Lead-Foot Driving vs. Feather-Foot Driving
Result: Major savings potential.
This is gonna hurt. From all our testing, the most successful method for saving gas is: you. And we're talking massive fuel economy gains. Think you need a hybrid? Well, chances are you've got hybrid-style mileage improvements already in your gas pedal foot. Don't mash the gas pedal when you stop and start. Take the long view of the road and brake easy. This tip alone can save you unbelievable amounts of gas. We found that if you slowed your 0-to-60 time down to 20 seconds from a normal city driving pace of 10 to 15 seconds, you'll feel the savings immediately.
Method: We drove the same loops as before, once by accelerating aggressively 15 times at three-fourths throttle from zero to a cruising speed of 75 mph. We also applied the brakes hard as if coming to a stoplight. In the second loop we accelerated moderately 15 times at one-fourth throttle to a cruising speed of 70 mph. We braked lightly to a full stop.
With lead foot 14.1 mpg
With feather foot 19.1 mpg
Percent change 35.4 percent improvement
With lead foot 18.1 mpg
With feather foot 23 mpg
Percent change 27.1 percent improvement
Test #4: Low Tire Pressure vs. Properly Inflated Tires
Result: Important for many reasons
No matter how many times drivers hear that tire pressure is important they hate checking it. Probably because they don't like squatting beside their car in a busy gas station with fumes swirling around them. But it is important, for a number of reasons. Properly inflated tires are less likely to fail at high speeds. They wear more evenly and, yes, they deliver better gas mileage. How much? In this test we saw a modest but noticeable difference. It might have been more dramatic if the test was conducted at a lower temperature; it was 108 degrees Fahrenheit the day we did our tests.
Method: We drove two loops at about 60 mph. Once, the tires were 5 psi below the pressure recommended by the manufacturer. The second time, the tires were about 2 psi above the recommended around. We overinflated the tires because it's difficult to get a reliable inflation level when the tires are already warmed up. We felt that it was important to make sure the tires were inflated to the recommended level or slightly above. Furthermore, tires are constructed to allow for a certain amount of overinflation, though they will then deliver a harsher ride.
Underinflated tires 20.5 mpg
Properly inflated tires 21.4 mpg
Percent change 4.4 percent improvement
Underinflated tires 23.7 mpg
Properly inflated tires 23.7 mpg
Percent change 0 percent improvement *
* We felt that because of the high temperatures the tires were never sufficiently underinflated enough to show a difference. Even though we deflated them by 5 psi when cold, the high temperature brought them back up to the required inflation level.
The good news is that you can drastically improve your gas mileage. The caveat is that you have to change your driving habits. Basically, stop driving like a maniac and use that cruise control. Who knows? You might like the new you.
On other fronts, be sure to check your tire inflation both to save gas but also to be safe and promote even tire wear. And regarding that air conditioner, well, if you want to drive around with your arm hanging out of the window it won't really change your gas mileage much.
Now, when someone sidles up to you at a party and says, "You know, washing your car will improve the gas mileage,'" you can lock eyes with them and say, "I heard that, too. Turns out the difference is so small it's insignificant."